In the manufacture of products, a wide variety of treating operations are known and used for imparting particular aesthetic or functional features to certain components of such products. For example, electroplating and electroless plating and other techniques can be used to deposit a metal coating upon a substrate. Instead of metal coatings, powdered organic or inorganic materials can be deposited upon such substrates. Etching, pickling, passivating, oxidizing or similar techniques can be utilized for altering the surface of the parts or for preparation prior to a subsequent coating operation. In addition, it is often necessary to clean, dry, rinse, wash, soak or otherwise process such parts to eliminate traces of preceding treatments or remove undesirable foreign matter therefrom.
When the parts to be treated are relatively large in size, any of the previously mentioned operations can be carried out fairly easily. When large numbers of small parts require such treatments, however, various problems can arise, particularly when all accessible surfaces of such parts are to obtain with certainty a high degree of uniformity through such treatments.
The term "treatment" is used to mean that the entire part including all accessible internal and external surfaces is changed, modified or affected. As noted above, typical treatments include electroplating, electroless plating, etching, pickling, passivating, reducing, rinsing, washing, drying, soaking, cleaning, coating, diffusion and the like. As one skilled in the art will appreciate, these treatments all have a common basis in that some type of treatment medium (i.e., a fluid) must come in contact with the surfaces of the part to be treated. Often, the fluid is a liquid, such as an aqueous or organic solution which may be acidic or basic with or without the addition of other chemicals or agents therein. It is also contemplated that the fluid be a gas or mixture of gasses.
In order to obtain uniformly treated parts, the ideal situation would be to process one part at a time. For very large numbers of small precision parts, such as ball point pen tips, for example, it is impractical and virtually impossible to economically employ an individual treatment for each part. Despite their small sizes, such parts are provided with bored openings, narrow gaps and grooves of very high dimensional precision, constituting the ball socket and the ink feed grooves as well as the holes connecting with the ink supply, all of which are necessary for proper operation of the pens in which they are utilized. Due to the large quantity of such small parts, a batch or heap process must be used to treat the surfaces of these parts, whereby a large number of parts, in the form of a large randomly oriented mass which is retained in an enclosure, is introduced into the treating environment.
Due to the contact between the parts and the packing thereof due to their weight, as well as due to the capillary nature of features incorporated in such parts, it is virtually impossible to obtain a uniform treatment of all surfaces of each part. In a situation where such parts are to be electroless nickel plated, for example, it is virtually impossible to assure that all surfaces of each part will experience the same degree of contact and interaction with the plating solution. This results in some portions of the part surface receiving a thinner deposit while other portions receive the appropriate thickness. If the process is continued until all accessible inner and outer surfaces of the parts have, as a minimum, the appropriate thickness, then certain portions of some surfaces will undoubtedly have a greater thickness than calculated. In either situation, the resultant plating is not uniform, with problems occurring because a very high degree of uniformity is required for such parts.
In addition, liquids are retained between the parts due to the varying degree of capillary attraction or adhesion. Such forces of capillary nature are combined with the forces attributable to the angle of contact at the interface of the various treatment mediums and the surfaces of parts to be treated, and further compounded by gas bubbles obstinately resisting displacement by the treatment medium. Such forces restrict the flow of fluid into, through and around the parts, which further contributes to nonuniform and unpredictable results of the treatment of holes, apertures, slots, or other narrow passages relative to other surfaces which are more readily accessible for contact with the treatment medium.
For the electroless nickel plating of such small parts, the volume of the electrolyte containing the nickel compound would be in a varying degree deprived of its active potential as it passes into, between, through or around the parts. This results in a thin deposit on some surfaces while the more accessible surfaces on the outermost parts will interact with electrolyte having a normal potency rate so that a thicker deposit will be achieved there. Surfaces which experience a normal exchange rate would have a predictable deposit thickness proportional to the time the parts are immersed in the electroless nickel bath. If the thickness of deposit on these accessible surfaces is desired to be thin (i.e., on the order of about 2 to 3 micrometers), the coating throughout small openings may be insufficient to allow a coherent, non-porous deposit which is required to achieve corrosion shielding.
Various methods have been attempted in an effort to increase the exchange of liquids through the narrow passages between small parts which are randomly oriented in a heap or other randomly oriented aggregate of such parts. This is of importance when the contact between the objects and a liquid must be of controlled duration in order to obtain a uniform result through the interaction between the surfaces and the liquid, e.g. when etching, plating or rinsing operations are to be carried out and a high degree of uniformity is to be achieved.
According to the prior art, small parts or objects are subjected to prolonged tumbling, vibration or agitation in an attempt to achieve a uniform treatment. These methods, which comprise ultrasonic agitation, vibration, barrel tumbling and the like, possess an inherent disadvantage in that they deleteriously affect or damage sharp edges or fragile thin sections of the parts. In addition, when the surfaces of the parts to be treated are openings such as through-holes, blind-holes or other narrow passages in the parts, the capillary forces (e.g. adhesion, angle of contact, viscosity and surface tension) again would prevent or inhibit the movement and exchange of liquids relative to the totality of the surfaces. These known methods only moderately increase the relative flow but do not overcome flow inhibitions such as those caused by the resistance of small gas bubbles against being displaced or the earlier mentioned capillary attraction forces. The present invention provides a method and apparatus for substantially reducing these problems so that a uniform treatment of all surfaces of such parts can be achieved.